The present invention relates to a method of generating light pulses in an ultrawide band ranging from the near-infrared region to the near-ultraviolet region, through utilization of interaction between laser light pulses and a nonlinear optical material.
Conventionally, widening of the spectrum of light pulses has been achieved through an operation of passing a single light pulse train having a predetermined center wavelength through a nonlinear optical material.
This method utilizes the self phase modulation effect, such that the refraction index of a nonlinear optical material changes with the intensity of the pulse train itself, and thus, the phase of the pulse train is modulated.
The frequency band of 230 THz (wavelength: 600 nm to 1100 nm) has been used in both the case in which a quartz fiber is used [A. Baltuska, Z. Wei, M. S. Pshenichnikov, D. A. Wiersma, and R. Szipocs, Appl. Phys, B65, 175 (1997)] and in the case in which a hollow glass fiber filled with rare gas such as argon or krypton is used [M. Nisoli, S. Stagira, S. De Silvestri, O. Svelto, S. Sartania, Z. Cheng, M. Lenzner, Ch. Spielmann, and F. Krausz, Appl. Phys, B65, 189 (1997)].
When the spectrum band is widened beyond the above-described band by the above-described conventional methods utilizing self phase modulation only, the intensity of light pulses must be increased beyond a damage limit of a nonlinear optical material used, raising the problem of damage being inflicted on the medium. Therefore, widening of the spectrum band has been difficult to achieve.
An object of the present invention is to solve the above-described problem and to provide an ultrawide-band light pulse generation method of generating light pulses in an ultrawide band ranging from the near-infrared region to the near-ultraviolet region.
In order to achieve the above object, the present invention provides the following.
[1] A method of generating light pulses in an ultrawide band, the method comprising the steps of: causing an ultrashort pulse laser serving as a light pulse source to emit sub-picosecond light pulses (hereinafter referred to as xe2x80x9cfundamental pulsesxe2x80x9d); passing the fundamental pulses through at least one nonlinear optical material in order to obtain light pulses having a center wavelength different from that of the fundamental pulses (hereinafter referred to as xe2x80x9cwavelength-converted pulsesxe2x80x9d); separating the wavelength-converted pulses from the fundamental pulses; delaying the wavelength-converted pulses relative to the fundamental pulses; adjusting the polarization of the wavelength-converted pulses or the fundamental pulses; adjusting the energy of the fundamental pulses and the energy of the wavelength-converted pulses; superposing the separated fundamental and wavelength-converted pulses on each other; and causing the superimposed pulses to enter, propagate through, and exit a nonlinear optical member.
[2] A method of generating light pulses in an ultrawide band according to [1], wherein the wavelength-converted pulses are generated from the fundamental pulses by sole or combined use of optical parametric oscillation-amplification, stimulated Raman scattering, stimulated Brillouin scattering, and single-filament continuous wave generation, all of which involve harmonic generation, sum frequency generation, and difference frequency generation.
[3] A method of generating light pulses in an ultrawide band according to [1], wherein the wavelength-converted pulses are generated from a laser having a resonator independent of that of the laser used for generation of the fundamental pulses; and feedback control is effected to maintain a constant relative phase difference between the fundamental pulses and the wavelength-converted pulses.
[4] A method of generating light pulses in an ultrawide band according to [1], wherein in the final step of introducing the fundamental pulses and the wavelength-converted pulses into the nonlinear optical member to thereby superimpose them together, the fundamental pulses and the wavelength-converted pulses are superimposed on each other in the vicinity of the terminal end of a waveguide path of the nonlinear optical member.
[5] A method of generating light pulses in an ultrawide band according to [1], wherein the wavelength-converted pulses are second-harmonic pulses which are obtained through passage of the fundamental pulses through the nonlinear optical material.
[6] A method of generating light pulses in an ultrawide band according to [1], wherein the source of the fundamental pulses is a fiber laser, a semiconductor laser, a solid-state laser, or a combination of one of these lasers and an amplification system.
[7] A method of generating light pulses in an ultrawide band according to [1], wherein the nonlinear optical member through which the fundamental pulses and the wavelength-converted pulses propagate in the final step is an optical fiber selected from the group of consisting of quartz fiber, organic fiber, and polymer fiber.
[8] A method of generating light pulses in an ultrawide band according to [1], wherein the nonlinear optical member through which the fundamental pulses and the wavelength-converted pulses propagate in the final step is a hollow fiber filled with a gas.
[9] A method of generating light pulses in an ultrawide band according to [1], wherein the nonlinear optical member through which the fundamental pulses and the wavelength-converted pulses propagate in the final step is an optical modulation member having nonlinear optical characteristics which assumes the form of bulk, thin layer, film, or photonic crystal structure.